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  <title>Document</title>
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  <script>
    let arr =[1,2,3,4]
    // 末尾新增  push  后面跟的是括号，再写新元素   
      arr.push (0)
      console.log(arr);//    打印数组并且返回数组长度
    //  开头追加     unshift
  </script>
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